This invention relates to a photobleachable dye composition. The present invention also relates to an imaging element containing the composition, and a method for photobleaching of a photographic element containing the composition.
In the field of photography, it is known to provide photographic elements comprising filter or antihalation compositions to improve the resolution of photographic materials. These antihalation dye compositions can be in the emulsion layer, or preferably in a non-image forming auxiliary layer. Without these antihalation dye layers, radiation reaching the photographic emulsion layer would be reflected, and image sharpness would decrease. After the image-wise exposure of the photographic element, the antihalation dye composition is generally bleached during or after the processing. Thermally bleachable antihalation dye compositions are known. Photobleachable antihalation dye compositions and chemically bleachable antihalation dye compositions are also known wherein subsequent to the imaging and processing steps, the film could then be exposed to a suitable light source or chemicals to bleach the dye.
For example, U.S. Pat. No. 4,548,896 discloses an imagewise bleachable composition comprising a dye in reactive association with a mesoionic compound, the composition being bleached through exposure to radiation at a wavelengths between 200 and 1000 nm.
U.S. Pat. No. 3,745,009 discloses a bleachable dye wherein an N-oxy substituent is part of the dye chromophore. The synthesis of these dyes is usually complex. Further, these dyes need to be carefully handled. Due to their inherent bleachable ability, they are not light stable.
U.S. Pat. Nos. 4,743,528, 4,743,529, 4,743,530 and 4,743,531 disclose a polymerisable composition comprising an azinium activator, a photosensitizer, and an ethylenically unsaturated monomers.
EP 308274 discloses a photobleachable cyanine dye composition containing a borate salt. The technology appears to be useful with a very limited variety of bleachable dyes.
Many of the known bleaching methods suffer from one or more disadvantages. They are limited to either heat or light sensitivity or to the required numbers of steps for carrying out the bleaching. Some methods are also limited to the narrow number of useful dyes, or by the effort required for synthesizing the useful dye.
It is desirable to find a photobleachable composition that eliminates the drawbacks of the known photobleachable compositions.
It is also desirable to provide a photographic element containing a photobleachable composition and a method for bleaching the photographic element containing the photobleachable composition.
These and other objects are achieved by the present invention, which provides a visible-light sensitive photobleachable dye composition, substantially free of polymerisable monomer, comprising a photobleachable dye and an N-oxyazinium compound.
The invention also relates to an imaging element comprising a support having thereon at least one image forming layer, and at least one non-image forming layer wherein the element further comprises a visible-light sensitive photobleachable composition containing a photobleachable dye and an N-oxyazinium compound.
Then, the invention relates to a method for bleaching a photographic element comprising a support having thereon at least one image forming layer, at least one non-image forming layer comprising a visible-light sensitive photobleachable composition containing a photobleachable dye and an N-oxyazinium compound, the method comprising:
exposing and processing the photographic element, and exposing the exposed and processed element, to radiation that can be absorbed either by the photobleachable dye or by the N-oxyazinium compound.
The method involves photochemically bleaching the photobleachable dyes relying on photoreactions of the photobleachable dye with an N-oxyazinium compound.
This invention provides a photobleaching method that can be advantageously carried out for a wide variety of photobleachable dyes such as sensitizing dyes, filter dyes, image dyes, infrared dyes or antihalation dyes. In addition, in the present invention, the photobleachable dye and the N-oxyazinium are distinct entities. As a result, the dyes can be easily handled in light, since the bleaching effect occurs only in the presence of the N-oxyazinium.
When reference in this application is made to a particular group, unless otherwise specifically stated, the group may itself be unsubstituted or substituted with one or more substituents (up to the maximum possible number). For example, xe2x80x9calkylxe2x80x9d group refers to a substituted or unsubstituted alkyl group, such as aralkyl group or sulfoalkyl group while xe2x80x9carylxe2x80x9d group refers to a substituted or unsubstituted aryl group (with up to six substituents) such as alkaryl or sulfoaryl group. The substituent may be itself substituted or unsubstituted.
Generally, unless otherwise specifically stated, substituents include any substituents, whether substituted or unsubstituted, which do not destroy properties necessary for the photographic utility. Examples of substituents include known substituents, such as: halogen, for example, chloro, fluoro, bromo, iodo; alkoxy, particularly those xe2x80x9clower alkylxe2x80x9d (that is, with 1 to 6 carbon atoms, for example, methoxy, ethoxy; substituted or unsubstituted alkyl, particularly lower alkyl (for example, methyl, trifluoromethyl); thioalkyl (for example, methylthio or ethylthio), particularly either of those with 1 to 6 carbon atoms; substituted and unsubstituted aryl, particularly those having from 6 to 20 carbon atoms (for example, phenyl); and substituted or unsubstituted heteroaryl, particularly those having a 5 or 6-membered ring containing 1 to 3 heteroatoms selected from N, O, or S (for example, pyridyl, thienyl, furyl, pyrrolyl); acid or acid salt groups such as any of those described below; and others known in the art. Alkyl substituents may specifically include xe2x80x9clower alkylxe2x80x9d (that is, having 1-6 carbon atoms), for example, methyl, ethyl, and the like. Further, with regard to any alkyl group or alkylene group, it will be understood that these can be branched or unbranched and include ring structures.
In the scope of the invention, the N-oxyazinium compound is an N-oxy-N-heterocyclic compound having a heterocyclic nucleus, such as a pyridinium, diazinium, or triazinium nucleus. The N-oxyazinium compound can include one or more aromatic rings, typically carbocyclic aromatic rings, fused with the N-oxy-N-heterocyclic compound, including quinolinium, isoquinolinium, benzodiazinium, phenanthridium and naphthodiazinium. Any convenient charge balancing counter-ion can be employed to complete the N-oxyazinium compounds. The oxy group (xe2x80x94Oxe2x80x94R,) of the N-oxyazinium compound which quaternizes the ring nitrogen atom of the azinium nucleus can be selected from among a variety of synthetically convenient oxy groups. The group R1 can, for example, be an alkyl group such as methyl, ethyl, butyl, benzyl, an aralkyl group (e.g., benzyl or phenethyl) and a sulfoalkyl group (e.g., sulfomethyl). The group R1 can be an aryl group such as a phenyl group. In another form R1 can be an acyl group, such as an xe2x80x94C(O)xe2x80x94R3 group, where R3 is an alkyl and aryl groups such as phenyl or naphthyl, tolyl, xylyl, etc. When R1 is an alkyl group, it typically contains from 1 to 18 carbon atoms, when R1 is an aryl group, it typically contains from 6 to 18 carbon atoms.
Illustrative examples of useful N-oxyazinium compounds are shown by the formulae below: 
wherein R1 represents alkyl group of 1-12 carbons, or alkyl group substituted with one or more groups selected from the group consisting of acyloxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl, aryloxycarbonyl, acetyl, aroyl, alkylaminocarbonyl, arylaminicarbonyl, alkylaminocarbonyloxy, alkylaminocarbonyloxy, acylamino, carboxy, sulfo, trihalomethyl, alkyl, aryl, heteroaryl, alkylureido, arylureido, succinimido, and phthalimido substituent; aryl group, or acyl group; each R2, R22 and R6 represents independently hydrogen, an alkyl group of 1-12 carbons, an aryl or heteroaryl group, unsubstituted or substituted with one or more substituents selected from the group consisting of an acyloxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl, aryloxycarbonyl, acetyl, aroyl, alkylaminocarbonyl, arylaminicarbonyl, alkylaminocarbonyloxy, arylaminocarbonyloxy, acylamino, carboxy, sulfo, trihalomethyl, alkyl, aryl, heteroaryl, alkylureido, arylureido, succinimido and phthalimido substituent, or an acyloxy, hydroxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl, aryloxycarbonyl, acetyl, aroyl, alkylaminocarbonyl, arylaminicarbonyl, alkylaminocarbonyloxy, arylaminocarbonyloxy, acylamino, amino, alkylamino, arylamino, carboxy, sulfo, trihalomethyl, alkyl, aryl, heteroaryl, alkylureido, arylureido, succinimido, phthalimido group, xe2x80x94COxe2x80x94R3 wherein R3 is an alkyl or an aryl group, or xe2x80x94(CHxe2x95x90CH)mxe2x80x94R4 wherein R4 is an aryl or heterocyclic group; m is 1 or 2; Y is selected from the group consisting of S, O, Se, xe2x80x94C(R1)2, and xe2x80x94NR1; X is a divalent linking group selected from a group consisting of substituted or unsubstituted methylenes, (xe2x80x94CR5R7xe2x80x94)n and [(xe2x80x94CR5R7)nxe2x80x94X1xe2x80x94(CR5R7xe2x80x94)P] wherein R5 or R7 are independently hydrogen, alkyl, substituted alkyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl group, n and p are from 1-12, X1 is aryl or heteroaryl nuclei, carbonyl, sulfo, thio, oxy; and Z is an alkylidene group.
In the scope of the invention, each of the above formulae can comprise one or more R2, R22 or R6 groups.
Useful N-oxyazinium compounds can also be represented by the following formula 
wherein A+ is the N-oxyazinium moiety. The linking alkyl chain can have additional substituents, e.g., ether, ester, amide, etc.
According to one embodiment, the N-oxyazinium compound is a compound having one of the following formulae: 
wherein R1 is an alkyl, an aryl or an acyl, R2 or R22 are independently a hydrogen atom, alkyl, aryl, heterocyclic, carboxylic, carboxylate, carbonamido, sulfonamido, nitryl, groups, xe2x80x94COxe2x80x94R3 wherein R3 is an alkyl group or aryl group, or xe2x80x94CHxe2x95x90CH)mxe2x80x94R4 group wherein R4 is an aryl or heterocyclic group; X is an alkylene group, preferably xe2x80x94CH2)nxe2x80x94 wherein n is from 1 to 12.
According to a specific embodiment, R1 is preferably an alkyl having from 1 to 18 carbon atoms or an aryl group having from 6 to 18 carbon atoms.
Illustrative examples of N-oxyazinium compounds useful in the present invention are one of the following compounds:
According to a preferred embodiment, the N-oxyazinium compound has a reduction potential less negative than xe2x88x921.4 V, and comprises an N-oxy group capable of releasing an oxy radical that reacts with the photobleachable dye to produce bleached compound. In this patent application, reduction potentials are reported as xe2x80x9cVxe2x80x9d which represents xe2x80x9cvolts versus a saturated calomel reference electrodexe2x80x9d. Reduction potentials of N-oxyazinium compounds can be measured by standard methods well known to those of skill in the art.
The composition of the invention can contain one or more of any of the N-oxyazinium compounds disclosed therein.
The N-oxyazinium compounds are associated with a counter ion that is not involved in the activity of the present composition and can be any of the conventional anions, e.g., halide, fluoroborate, hexafluorophosphate, toluene sulfonate, etc. It can also be an oligomeric or polymeric species.
In the scope of the invention, the photobleachable dye is any dye that by reaction with an N-oxyazinium compound gives a bleached compound. According to the invention, a bleached compound is a colorless compound or a compound less colored than the dye.
The photobleachable composition is substantially free of polymerizable monomer, for example polymerizable monomer preferably is present in an amount less than 10% of the photobleachable composition, more preferable less than 5% and most preferably less than 1%.
The photobleachable dyes useful in the invention can be for example, cyanine dyes, complex cyanine dyes, merocyanine dyes, complex merocyanine dyes, homopolar cyanine dyes, styryl dyes, oxonol dyes, hemioxonol dyes, and hemicyanine dyes, squarilium dyes, coumarin dyes, rhodamine dyes, acridine dyes, oxanol dyes etc. Representative bleachable dyes are discussed in Research Disclosure, Item 36544, September 1996, the disclosure of which, including the disclosure of references cited therein are incorporated herein by reference. These dyes may be synthesized by those skilled in the art according to the procedures described herein or F. M. Hamer, The Cyanine Dyes and Related Compounds (Interscience Publishers, New York, 1964).
Photobleachable cyanine or merocyanine dyes can be represented by the general formulae D1-D5 below: 
wherein:
E1 and E2 represent the atoms necessary to form a substituted or unsubstituted hetero ring and may be the same or different,
each J independently represents a methine group,
q is a positive integer of from 1 to 4,
p and r each independently represents 0 or 1,
D1 and D2 each independently represents alkyl or aryl groups, and
W2 is a counterion as necessary to balance the charge; 
wherein E1, D1, J, p, q and W2 are as defined above for formula D1 and G represents 
wherein E4 represents the atoms necessary to complete a substituted or unsubstituted heterocyclic nucleus, and F and F1 each independently represents a cyano group, an ester group, an acyl group, a carbamoyl group or an alkylsulfonyl group; 
wherein D1, E1, J, p, q and W2 are as defined above for formula D1, and G2 represents an amino group or a substituted or unsubstituted aryl group; 
wherein D1, E1, D2, E1, J, p, q, r and W2 are as defined for formula D1 above, and E3 is defined the same as E4 for formula D2 above; 
wherein D1, E1, J, G, p, q, r, W2 and E3 are as defined above.
In the above formulas, E1 and E2 each independently represents the atoms necessary to complete a substituted or unsubstituted 5- or 6-membered heterocyclic nucleus. These include a substituted or unsubstituted: thiazole nucleus, oxazole nucleus, selenazole nucleus, quinoline nucleus, tellurazole nucleus, pyridine nucleus, thiazoline nucleus, indoline nucleus, oxadiazole nucleus, thiadiazole nucleus, or imidazole nucleus. This nucleus may be substituted with known substituents, such as halogen (e.g., chloro, fluoro, bromo), alkoxy (e.g., methoxy, ethoxy), substituted or unsubstituted alkyl (e.g., methyl, trifluoromethyl), substituted or unsubstituted aryl, substituted or unsubstituted aralkyl, sulfonate, and others known in the art.
In one embodiment of the invention, when dyes according to formula D1 are used E1 and E2 each independently represent the atoms necessary to complete a substituted or unsubstituted thiazole nucleus, a substituted or unsubstituted selenazole nucleus, a substituted or unsubstituted imidazole nucleus, or a substituted or unsubstituted oxazole nucleus.
Examples of useful nuclei for E1 and E2 include: a thiazole nucleus, e.g., thiazole, 4-methylthiazole, 4-phenylthiazole, 5-methylthiazole, 5-phenylthiazole, 4,5-dimethyl-thiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, benzothiazole, 4-chlorobenzothiazole, 5-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, 5-methylbenzothiazole, 6-methylbenzothiazole, 5-bromobenzothiazole, 6-bromobenzothiazole, 5-phenylbenzothiazole, 6-phenylbenzothiazole, 4-methoxybenzothiazole, 5-methoxybenzothiazole, 6-methoxybenzothiazole, 4-ethoxybenzothiazole, 5-ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, 5,6-dioxymethylbenzothiazole, 5-hydroxybenzothiazole, 6-5-dihydroxybenzothiazole, naphtho[2,1-d]thiazole, 5-ethoxynaphtho[2,3-d]thiazole, 8-methoxynaphtho[2,3-d]thiazole, 7-methoxynaphtho[2,3-d]thiazole, 4xe2x80x2-methoxythianaphtheno-7xe2x80x2,6xe2x80x2-4,5-thiazole, etc.; an oxazole nucleus, e.g., 4-methyloxazole, 5-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, 5-phenyloxazole, benzoxazole, 5-chlorobenzoxazole, 5-methylbenzoxazole, 5-phenylbenzoxazole, 6-methylbenzoxazole,, 5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, 5-ethoxybenzoxazole, 5-chlorobenzoxazole, 6-methoxybenzoxazole, 5-hydroxybenzoxazole, 6-hydroxybenzoxazole,, naphtho[2,1-d]oxazole, naphtho[1,2-d]oxazole, etc.; a selenazole nucleus, e.g., 4-methylselenazole, 4-phenylselenazole, benzoselenazole, 5-chlorobenzoselenazole, 5-methoxybenzoselenazole, 5-hydroxybenzoselenazole, tetrahydrobenzoselenazole, naphtho[2,1-d]selenazole, naphtho[1,2-d]selenazole, etc.; a pyridine nucleus, e.g., 2-pyridine, 5-methyl-2-pyridine, 4-pyridine, 3-methyl-4-pyridine, 3-methyl-4-pyridine, etc.; a quinoline nucleus, e.g., 2-quinoline, 3-methyl-2-quinoline, 5-ethyl-2-quinoline, 6-chloro-2-quinoline, 8-chloro-2-quinoline, 6-methoxy-2-quinoline, 8-ethoxy-2-quinoline, 8-hydroxy-2-quinoline, 4-quinoline, 6-methoxy-4-quinoline, 7-methyl-4-quinoline, 8-chloro-4-quinoline, etc.; a tellurazole nucleus, e.g., benzotellurazole, naphtho[1.2-d]benzotellurazole, 5,6-dimethoxybenzotellurazole, 5-methoxybenzotellurazole, 5-methylbenzotellurazole; a thiazoline nucleus, e.g.,thiazoline, 4-methylthiazoline, etc.; a benzimidazole nucleus, e.g., benzimidazole, 5-trifluoromethylbenzimidazole, 5,6-dichlorobenzimidazole; and indole nucleus, 3,3-dimethylindole, 3,3-diethylindole, 3,3,5-trimethylindole; or a diazole nucleus, e.g., 5-phenyl-1,3,4-oxadiazole, 5-methyl-1,3,4-thiadiazole.
F and Fxe2x80x2 are each a cyano group, an ester group such as ethoxy carbonyl, methoxycarbonyl, etc., an acyl group, a carbamoyl group, or an alkylsulfonyl group such as ethylsulfonyl, methylsulfonyl, etc. Examples of useful nuclei for E4 include a 2-thio-2,4-oxazolidinedione nucleus (i.e., those of the 2-thio-2,4-(3H,5H)-oxaazolidinone series) (e.g., 3-ethyl-2-thio-2,4 oxazolidinedione, 3-(2-sulfoethyl)-2-thio-2,4 oxazolidinedione, 3-(4-sulfobutyl)-2-thio-2,4 oxazolidinedione, 3-(3-carboxypropyl)-2-thio-2,4 oxazolidinedione, etc.; a thianaphthenone nucleus (e.g., 2-(2H)-thianaphthenone, etc.), a 2-thio-2,5-thiazolidinedione nucleus (i.e., the 2-thio-2,5-(3H,4H)-thiazoledeione series) (e.g., 3-ethyl-2-thio-2,5-thiazolidinedione, etc.); a 2,4-thiazolidinedione nucleus (e.g., 2,4-thiazolidinedione, 3-ethyl-2,4-thiazolidinedione, 3-phenyl-2,4-thiazolidinedione, 3-a-naphthyl-2,4-thiazolidinedione, etc.); a thiazolidinone nucleus (e.g., 4-thiazolidinone, 3-ethyl-4-thiazolidinone, 3-phenyl-4-thiazolidinone, 3-a-naphthyl-4-thiazolidinone, etc.); a 2-thiazolin-4-one series (e.g., 2-ethylmercapto-2-thiazolin-4-one, 2-alkylphenyamino-2-thiazolin-4-one, 2-diphenylamino-2-thiazolin-4-one, etc.) a 2-imino-4-oxazolidinone (i.e., pseudohydantoin) series (e.g., 2,4-imidazolidinedione (hydantoin) series (e.g., 2,4-imidazolidinedione, 3-ethyl-2,4-imidazolidinedione, 3-phenyl-2,4-imidazolidinedione, 3-a-naphthyl-2,4-imidazolidinedione, 1,3-diethyl-2,4-imidazolidinedione, 1-ethyl-3-phenyl-2,4-imidazolidinedione, 1-ethyl-2-a-naphthyl-2,4-imidazolidinedione, 1,3-diphenyl-2,4-imidazolidinedione, etc.); a 2-thio-2,4-imidazolidinedione (i.e., 2-thiohydantoin) nucleus (e.g., 2-thio-2,4-imidazolidinedione, 3-ethyl-2-thio-2,4-imidazolidinedione, 3-(2-carboxyethyl)-2-thio-2,4-imidazolidinedione, 3-phenyl-2-thio-2,4-imidazolidinedione, 1,3-diethyl-2-thio-2,4-imidazolidinedione, 1-ethyl-3-phenyl-2-thio-2,4-imidazolidinedione, 1-ethyl-3-naphthyl-2-thio-2,4-imidazolidinedione, 1,3-diphenyl-2-thio-2,4-imidazolidinedione, etc.); a 2-imidazolin-5-one nucleus.
G2 represents an amino group (e.g., primary amino, anilino), or an aryl group (e.g., phenyl, naphthyl, dialkylaminophenyl, tolyl, chlorophenyl, nitrophenyl).
According to the formulas D1-D5, each J represents a methine group. Examples of substituents for the methine groups include alkyl (preferably of from 1 to 6 carbon atoms, e.g., methyl, ethyl, etc.) and aryl (e.g., phenyl). Additionally, substituents on the methine groups may form bridged linkages.
W2 represents a counterion as necessary to balance the charge of the sensitizing dye. Such counterions include cations and anions for example sodium, potassium, triethylammonium, tetramethylguamidinium, diisopropylammonium and tetrabutylammonium, chloride, bromide, iodide, para-toluene sulfonate and the like.
D1 and D2 are each independently aryl groups (preferably of 6 to 15 carbon atoms), or more preferably, alkyl groups (preferably of from 1 to 6 carbon atoms). Examples of aryl include phenyl, tolyl, p-chlorophenyl, and p-methoxyphenyl. Examples of alkyl include methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl, dodecyl, etc., and substituted alkyl groups (preferably a substituted lower alkyl containing from 1 to 6 carbon atoms), such as a hydroxyalkyl group, e.g., 2-hydroxyethyl, 4-hydroxybutyl, etc., a carboxyalkyl group, e.g., 2-carboxyethyl, 4-carboxybutyl, etc., a sulfoalkyl group, e.g., 2-sulfoethyl, 3-sulfobutyl, 4-sulfobutyl, etc., a sulfatoalkyl group, etc., an acyloxyalkyl group, e.g., 2-acetoxyethyl, 3-acetoxypropyl, 4-butyroxybutyl, etc., an alkoxycarbonlyalkyl group, e.g., 2-methoxycarbonlyethyl, 4-ethoxycarbonylbutyl, etc.,or an aralkyl group, e.g., benzyl, phenethyl, etc.
According to a different embodiment, the photobleachable dye useful in the composition of the invention is an oxonol dye. Oxonol dyes can be a methine oxonol dye having the following formula: 
wherein n is 0, 1, or 2; R1, R2, R3, and R4 are the same or different alkyl or aryl groups, one or more of R1, R2, R3, and R4 contains carboxy substituent xe2x80x94CO2Z, wherein Z+ is a statistical mixture of hydrogen (H+) and alkali or tetraalkylammonium cations (M+) such that Z+=xH++(1xe2x88x92x)M+, where x is a decimal ranging from about 0.33 to about 0.95. These dyes have been disclosed in details in U.S. Pat. No. 5,274,109, incorporated therein by reference.
Other oxonols useful in the present invention can be any oxonol dyes disclosed in U.S. Pat. Nos. 4,877,721, 5,674,669, EP 740,200, EP 549,486, EP 246,553, incorporated therein by reference.
The photobleachable dyes can also be a photographic filter dye of the structure 
wherein A represents a nucleus having a carboxyphenyl or sulfonamidophenyl substituent selected from the group consisting of 2-pryazolin-5-ones free of any substituent bonded thereto through a carboxyl group, rhodanines, hydantoins, 2-thiohydantoins, 4-thiohydantoins, 2,4-oxazolidindiones, 2-thio-2,4-oxazolidindiones, isoxazolinones, barbiturics, 2-thiobarbiturics, and indandiones, R represent hydrogen, substituted or unsubstituted alkyl of 1 to 4 carbon atoms, or benzyl, R1 and R2 each independently represents substituted or unsubstituted alkyl or aryl, or taken together with R5, R6, N and the carbon atoms to which they are attached, represent the atoms needed to complete a julolydyl ring, R3 represents H, or substituted or unsubstituted alkyl or aryl, R5 and R6 each independently represents H, or R1 taken together with R1, or R6 taken together with R2, represent the atoms necessary to complete a carbocyclic ring such as tetrahydroquinoyl, and m is 0 or 1. These filter dyes have been disclosed in U.S. Pat. No. 4,857,446, incorporated therein by reference.
Bleachable dyes can be azomethine dyes as disclosed in U.S. Pat. No. 5,177,052, or U.S. Pat. No. 5,871,890, incorporated therein by reference.
The photobleachable dyes useful in the composition of the present invention can also be is squarilium dyes. The squarilium can have the following formula: 
wherein: R1, R2, R3 and R4 each independently represents hydrogen, hydroxy, halogen, cyano, alkoxy, aryloxy, acyloxy, aryloxycarbonyl, alkoxycarbonyl, sulfonyl, carbamoyl, acyl, acylamido, alkylamino, arylamino or a substituted or unsubstituted alkyl, aryl or hetaryl group; or any of said R1, R2, R3 or R4 groups may be combined with R5, R6, R7 or R8 or with each other to form a 5 to 7-membered substituted or unsubstituted carbocyclic or heterocyclic ring; R5, R6, R7 and R8 each independently represents hydrogen, a substituted or unsubstituted alkyl or cycloalkyl group having from 1 to about 6 carbon atoms or an aryl or hetaryl group having from about 5 to about 10 atoms; or R5 and R6 or R7 and R8 may be joined together to form a 5 to 7-membered substituted or unsubstituted nitrogen-containing heterocyclic ring; and n and m are each independently 1 to 4. These dyes have been disclosed in details in U.S. Pat. No. 4,942,141, incorporated therein by reference.
Examples of bleachable dyes useful in the invention are: 
The composition of the invention can further comprise additional additives such as plasticizers, solvents, or binders. According to a specific embodiment, the composition of the invention further comprises a polymeric binder such as a polyacrylate, polymethacrylate, poly(acrylate co methacrylate) or a mixture thereof.
The photographic elements useful in the present invention can be any known photographic elements such as black and white elements, single color elements or multicolor elements. Conventionally multicolor elements contain dye image-forming units sensitive to each of the three primary regions of the spectrum. Each unit can be comprised of a single emulsion layer or of multiple emulsion layers sensitive to a given region of the spectrum. The layers of the element, including the layers of the image-forming units, can be arranged in various orders as known in the art. In an alternative format, the emulsions sensitive to each of the three primary regions of the spectrum can be disposed as a single segmented layer.
A typical multicolor photographic element comprises a support bearing a cyan dye image-forming unit comprised of at least one red-sensitive silver halide emulsion layer having associated therewith at least one cyan dye-forming coupler, a magenta dye image-forming unit comprising at least one green-sensitive silver halide emulsion layer having associated therewith at least one magenta dye-forming coupler, and a yellow dye image-forming unit comprising at least one blue-sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler. The element can contain additional layers, such as filter layers, interlayers, overcoat layers, subbing layers, and the like. All of these can be coated on a support that can be transparent or reflective (for example, a paper support).
The composition of the invention can be contained in any of the layers of the photographic element. According to one embodiment, the element of the invention is a photothermographic element comprising conventionally a support, preferably a transparent support, having thereon at least one photosensitive layer and a antihalation layer. In the photothermographic element of the invention, the photosensitive layer can be a silver photosensitive layer or a non-silver photosensitive layer. Non photosensitive layer can be for example a layer containing a diazo compound. According to a specific embodiment, the composition containing the N-oxyazinium is incorporated in a non image-forming layer of the photographic element of the invention, preferably in an antihalation layer.
Photographic elements comprising the composition of the invention can be processed in any of a number of well-known photographic processes utilizing any of a number of well-known processing compositions, described, for example, in T. H. James, editor, The Theory of the Photographic Process, 4th Edition, Macmillan, N.Y. 1977. In the case of processing a negative working element, the element is treated with a color developer (that is one which will form the colored image dyes with the color couplers), and then with a oxidizer and a solvent to remove silver and silver halide. In the case of processing a reversal color element, the element is first treated with a black and white developer (that is, a developer which does not form colored dyes with the coupler compounds) followed by a treatment to fog silver halide (usually chemical fogging or light fogging), followed by treatment with a color developer. Preferred color developing agents are p-phenylenediamines. Development is followed by bleach-fixing, to remove silver or silver halide, washing and drying.
In the method of the invention, the photobleaching can be carried out by photoexcitation of the photobleachable dye (i.e., visible to infrared light, depending on the absorption range of the dye) or by photoexcitation of the N-oxyazinium (mostly ultraviolet light). The bleaching reaction is believed to occur as a result of reaction of an alkoxy radical that results from cleavage of the Nxe2x80x94O bond of the N-oxyazinium compound.
In the following reaction, the successive reactions are disclosed from a pyridinium compound as N-oxyazinium compound, however it should be understood that the useful compound can be any N-oxyazinium compounds useful in the scope of the invention.
In reactions induced by photoexcitation of a dye, it is believed that the excited dye (dye*) transfers an electron to the N-oxyazinium compound to yield an oxidized dye (a dye radical cation, dyexe2x80xa2+) and a reduced N-oxyazinium compound (the radical, Axe2x80xa2). The alkoxy radical (Axe2x80xa2) fragments to give an oxy radical (xe2x80xa2OR1) and a nitrogen heterocycle (A). Reaction of the oxy radical with the dye, or with the oxidized dye (dyexe2x80xa2+), leads to a colorless compound or a compound less colored than the dye thus providing a bleached compound. 
The feasibility of electron transfer from an excited dye to an N-oxyazinium compound depends on the energetics of the reaction. The reaction energetics are determined by the relative reduction potentials of the photobleachable dye and the N-oxyazinium compound. According to a preferred embodiment, the reduction potential of the N-oxyazinium compound is less negative than that of the photobleachable dye. However the reaction will still take place, although with a somewhat smaller rate constant, if the reduction potential of the N-oxyazinium compound is equal to or is slightly (ca. 0.1 V) more negative than that of the dye to be bleached.
For spectral sensitization of silver halide to occur efficiently using a sensitizing dye, the dye has to have a reduction potential which is either equal to or is more negative than ca xe2x88x920.9 V, vs. SCE (saturated calomel electrode). Thus, in this embodiment, any N-oxyazinium compound with a reduction potential less negative than ca. xe2x88x921.2 V would react with all sensitizing dyes.
For sensitizing dyes that have reduction potentials that are more negative than xe2x88x920.9 V, the range of the reduction potentials of the N-oxyazinium compounds can be extended in accordance with the general requirement mentioned above.
The reduction potential of the N-oxyazinium compounds can be measured by conventional electrochemical techniques. Alternatively, it can be estimated from the reduction potentials of the corresponding N-alkylazinium compounds that are reported in the literature, the reduction potential of N-oxyazinium compound being always less negative than the reduction potential of the corresponding N-alkylazinium compound. The N-oxyazinium compounds listed above have reduction potentials of xe2x88x920.9 V or less negative.
As mentioned above, to function as sensitizing dyes, these usually have reduction potentials of ca. xe2x88x920.9 V or more negative. Thus the energetic requirements mentioned above are met for any dye that is capable of sensitizing silver halide.
It is also believed that reactions via excitation of the N-oxyazinium compounds proceed via fragmentation of the Nxe2x80x94O bond of the photoexcited N-oxyazinium compound to yield the radical cation of the nitrogen heterocycle (Axe2x80xa2+) and an oxy radical (xe2x80xa2OR1). The radical cation (Axe2x80xa2+) can accept an electron from a dye to yield the nitrogen heterocycle (A) and the oxidized dye (a dye radical cation, dyexe2x80xa2+). Thus, the same intermediates are ultimately formed (xe2x80xa2OR1 and dyexe2x80xa2+) whether the reactions are initiated by dye excitation or by excitation of the N-oxyazinium compounds. 
In the other embodiment of the invention the N-oxyazinium salt can be photochemically excited, where bond cleavage yielding an oxy radical and the radical cation of the parent compound as mentioned above. It was found that this excitation mode could also lead to bleaching of the dyes. The energetic requirements mentioned above for reactions initiated by the photoexcited dye do not apply to the reactions initiated by photoexcitation of the N-oxyazinium compound. The latter reactions proceed via Nxe2x80x94O bond cleavage of the N-oxyazinium compound. The energetics of this reaction depend on the excitation energy of the N-oxyazinium compound and the Nxe2x80x94O bond dissociation energy. N-oxyazinium compounds with first absorption maxima in the UV range or around 400 nm have excitation energies far exceeding the energy required to break the Nxe2x80x94O bond.
In the method of the present invention, the exposed and processed photographic element is then exposed to radiation. Radiations that can be used are any radiation capable of producing the photobleaching of the dye. Radiations are selected according to the nature of the dye and for the N-oxyazinium compound. The method of the invention can be accomplished by a number of light sources. These include ambient room light from a fluorescent or incandescent lamps, from flash light, mercury or xenon light sources. The UV light may be filtered out by appropriate filters for selective exposure of the dyes, or unfiltered light may be used to excite both thedyes and the N-oxyazinium compounds. Alternatively, mostly UV light sources such as phosphor-coated low-pressure mercury lamps (300 to 350 nm) could be used. The exposure time varies from a few milliseconds when flash lamps are used to several tens of seconds when low intensity light sources are used.
When the composition of the invention contains a polymeric binder, it is preferred to carry out the method at a temperature higher than the glass transition of the polymeric binder present in the composition. If the glass transition temperature of the binder is above room temperature, exposure can be carried out at an elevated temperature or when lasers are used, the local temperature may rise to the desired level. Alternatively, an additive can be added to the composition to bring down the glass transition temperature of the composition. Such additives are well known in the art.